1. Field of the Invention
This invention relates to a transformer, and a method of assembling the same.
2. Prior Art
FIG. 1 shows a typical construction of a conventional transformer. In the figure, reference numeral 10 designates a bobbin which has a generally cylindrical shape and is made of plastics. The bobbin 10 has opposite ends thereof formed integrally with flanges 12 and a winding surface or outer peripheral surface 14 extending between the flanges 12, on which are wound winding layers. Specifically, a primary winding layer 16, a secondary winding layer 18, a secondary winding layer 20, a primary winding layer 22, and a secondary winding layer 24 are wound on the outer peripheral surface 14 in a fashion being superposed one upon another in the order mentioned from an inner side toward an outer side of the bobbin 10. The numbers of the primary and secondary winding layers are determined according to performance required of the transformer.
Interposed between the respective adjacent winding layers 16, 18, 20, 22, and 24 are interlayer tape layers 26, 28, 30 and 32 which are formed of adhesive tapes of an insulating material such as polyester to insulate the respective adjacent winding layers from each other. An outer peripheral tape layer 34 formed of a similar adhesive tape is applied on the outermost winding layer 24. In actuality, the tape layers 26 to 32 are wound on the respective inner winding layers in a plurality of turns dependent upon the required insulation performance and the safety standards.
Further, barrier tape layers 36, 38, 40 and 42 are provided at opposite sides of the respective winding layers 16, 18, 20 and 22. These barrier tape layers 36 to 42 are formed of adhesive tapes, e.g. of an insulating material such as a polyester tape and a non-woven composite tape of polyester impregnated with an epoxy resin.
The barrier tape layers serve to secure large required creeping distances between the respective adjacent winding layers 16, 18, 20 and 22. For example, by virtue of the presence of the barrier tape layers 36 and 38, the mutually adjacent winding layers 16 and 18 are spaced from each other along the interlayer tape layer 26 by a creeping distance corresponding to twice the widths of the barrier tapes, whereby it can be prevented that the winding layers 16 and 18 are shortcircuited to each other due to movement of a charge along the surfaces of the interlayer tape layer 26. Particularly, the barrier tape layers contribute to enhancement of the withstand voltage between a primary winding and a secondary winding such as the primary winding layer 16 and the secondary winding layer 18.
Requirements as to the creeping distance and the withstand voltage are prescribed by safety standards in various countries of the world to maintain the safety, etc. For example, in Japan, they are prescribed by Electrical Appliance And Material Law, Annexed List, Item C, in the United States, UL (Underwriters Laboratories Inc.)-1950, etc., and in Europe, IEC-EN60950, etc. Further, the numbers of turns in which tapes forming the barrier tape layers are to be wound are prescribed by the safety standards.
The conventional transformer with the barrier tape layers incorporated therein as above, however, is inevitably large in size. More specifically, the barrier tape layers 36 to 42 have substantial widths, which necessitates designing the bobbin 10 large in axial size. Besides, the barrier tape layers 36 to 52 are provided at the opposite sides of the respective winding layers 16 to 22 to positively prevent leakage of charge, and therefore the bobbin 10 has to have an axial size corresponding to twice the widths of the barrier tape layers 36 to 42.
In other words, the conventional transformer has a size larger than a size inherently required for the voltage transformation performance of the transformer. This makes it difficult to meet demands for development of information, communication, and broadcasting receiver apparatuses which are more compact in size.
Moreover, the safety standards vary between countries of the world. For example, according to UL-1950, the prescribed creeping distance is 3.2 mm or more, whereas, according to IEC-EN60950, it is 6.4 mm or more. Therefore, transformer manufacturers could not but take the following measures to cope with such differing standards.
(I) Design transformers according to the safety standards of each country and manufacture the thus designed transformers. This measure, however, entails increased manufacturing costs.
(II) Design all transformers according to the severest safety standards and manufacture the thus designed transformers. This measures, however, does not lead to final solution of the problem of large-sized transformers.
On the other hand, to solve the above problem, it has been proposed to coat a conductor forming the winding layer with a material having a high degree of insulation. This proposed method has the advantage that even if the creeping distance between the adjacent winding layers is made smaller, the withstand voltage does not decrease. To secure sufficient withstand voltage, however, the coating material having a high degree of insulation has to be coated on the conductor in a plurality of layers, or a single layer of the coating material which has an increased thickness has to be coated on the conductor. This results in an increased outer diameter of the entire conductor coated with the coating material, thus not substantially contributing to reduction of the size of the transformer. Besides, such a conductor coated with a material of a high degree of insulation is expensive. In addition, the coating material at ends of the conductor has to be removed in order to connect the conductor to terminals of the transformer, which is troublesome in assembling the transformer.